Abstract Metastasis is the major cause of death in breast cancer patients. However, the molecular mechanisms underlying tumor initiation and metastasis are not clear. We have recently identified tumor initiation cells (TICs) from human breast tumors. TICs carry certain properties of stem cells, are more resistant to conventional cancer therapies, and are involved in tumor metastasis. How to effectively target TICs or metastasis initiating cells (MICs) thus becomes one of the most propelling questions. Endogenous single strand small RNAs of 20- 22 nucleotides in length, known as microRNAs (miRNAs, miRs), have emerged to be powerful regulators of tumor progression. In this project, we aim to characterize novel miRs that regulate human breast cancer initiation and metastasis and identify their target genes. Then in our future endeavors, we will examine miR regulation mechanisms at the transcriptional level and further translate our understanding to clinical applications, such as novel cancer biomarkers or therapeutics. Most previous metastasis models are limited in their ability to fully represent human tumors, due to genetic changes accumulated in culture for human cancer cell lines, genetic differences in mouse tumor models compared to human tumors, and bypassing the natural steps of metastasis via bloodstream inoculations. This project will take advantage of our recently established human-in-mouse breast cancer models, which are derived from clinical tumor specimens and develop spontaneous lung or lymph node metastases upon orthotopic transplantation into mouse mammary fat pads. To closely monitor breast tumor initiation and metastasis in vivo, we have also transduced primary cancer cells with optical reporters and improved the detection sensitivity to 10 cells in vivo via non-invasive bioluminescence imaging. MiRs are more stable and resistant to analysis protocols than mRNAs, thus serving as promising novel cancer biomarkers. Furthermore, they are endogenous small RNAs with little toxicity compared to compound drugs; therefore hold the promise to be developed as innovative cancer therapeutics. Our long-term goal is to combine our understanding of tumor initiation and metastasis with knowledge in multi- disciplinary technology (such as chemistry and bioengineering) to improve clinical medicine and reduce cancer mortality.